http://chineseinput.net/에서 pinyin(병음)방식으로 중국어를 변환할 수 있습니다.
변환된 중국어를 복사하여 사용하시면 됩니다.
Hojun Cha,Youngjun Choi,Seokho Kim 한국초전도저온학회 2022 한국초전도저온공학회논문지 Vol.24 No.3
Liquid hydrogen (LH2) has a higher density than gaseous hydrogen, so it has high transport efficiency and can be stored atrelatively low pressure. In order to use efficient bulk hydrogen in the industry, research for the LH2 supply system is needed. In thehigh-pressure hydrogen station based on LH2 currently being developed in Korea, a heat exchanger is used to heat up supercriticalhydrogen at 700 bar and 60 K, which is pressurized by a cryogenic high-pressure pump, to gas hydrogen at 700 bar and 300 K. Accordingly, the heat exchanger used in the hydrogen station should consider the design of high-pressure tubes, miniaturization,and freezing prevention. A helical heat exchanger generates secondary flow due to the curvature characteristics of a curved tubeand can be miniaturized compared to a straight one on the same heat transfer length. This paper evaluates the heat transferperformance through parametric study on the distance between coils, guide effect, and anti-icing design of helical heat exchanger. The helical heat exchanger has better heat transfer performance than the straight tube exchanger due to the influence of thesecondary flow. When the distance between the coils is uniform, the heat transfer is enhanced. The guide between coils increasesthe heat transfer performance by increasing the heat transfer length of the shell side fluid. The freezing is observed around the inletof distribution tube wall, and to solve this problem, an anti-icing structure and a modified operating condition are suggested.
Evaluation of cryogenic mechanical properties of aluminum alloy using small punch test
Hojun Cha,Seungmin Jeon,Donghyeon Yoon,Jisung Yoo,Seunggun Lee,Seokho Kim 한국초전도저온학회 2023 한국초전도저온공학회논문지 Vol.25 No.4
The Small Punch Test (SPT) was developed to evaluate the softening and embrittlement of materials such as power plants and nuclear fusion reactors by taking samples in the field. Specimens used in the SPT are very thin and small disk-shaped compared to specimens for general tensile test, and thus have economic advantages in terms of miniaturization and repeatability of the test. The cryogenic SPT can also be miniaturized and has a significantly lower heat capacity than conventional universal test machines. This leads to reduced cooling and warm-up times. In this study, the cryogenic SPT was developed by modifying the existing room temperature SPT to be cooled by liquid nitrogen using a super bellows and a thermal insulation structure. Since the cryogenic SPT was first developed, basic experiments were conducted to verify the effectiveness of it. For the validation, aluminum alloy 6061-T6 specimens were tested for mechanical properties at room and cryogenic temperature. The results of the corrected tensile properties from the SPT experiment results were compared with known room temperature and cryogenic properties. Based on the correction results, the effectiveness of the cryogenic SPT test was confirmed, and the surface fracture characteristics of the material were analyzed using a 3d image scanner. In the future, we plan to conduct property evaluation according to the development of various alloy materials.
Jung Hoon Cha,Yong Jung Kim,Yong Cheon Cho,Hojun Lee,Hak Yong Lee 한국전자파학회JEES 2021 Journal of Electromagnetic Engineering and Science Vol.21 No.1
A meander microstrip antenna was designed to operate in the UHF and microwave bands. Laser direct structuring (LDS) and the flexible printed circuit board (FPCB) method were used to fabricate the antenna, and their results were compared. A dual-band low-power-consumption active radio frequency identification (RFID) tag based on the present antenna was developed and fabricated for an automated subway vehicle maintenance system. The recognition distance of the active tag is more than 20 m at 900 ㎒ and more than 170 m at 2.4 ㎓, while the recognition rate is >95% at 900 ㎒ and 100% at 2.4 ㎓. The 100% recognition rate was obtained when the RFID readers were placed in a line parallel to the subway track. Therefore, we have shown that our active RFID tag can be used in a subway vehicle management system.
Singil Kang,Hojun Cha,Seungcheol Ryu,Kiwhan Kim,Seungmin Jeon,Jaesun Lee,Seokho Kim 한국초전도저온학회 2022 한국초전도저온공학회논문지 Vol.24 No.1
Recently, research on applying composite materials to various industrial fields is being actively conducted. In particular, composite materials fabricated by Fused Deposition Modeling 3D printers have more advantages than existing materials as they have fewer restrictions on manufacturing shape, reduce the time required, weight. With these advantages, it is possible to consider utilizing composite materials in cryogenic environments such as the application of liquid oxygen and liquid hydrogen, which are mainly used in an aerospace and mobility. However, FDM composite materials are not verified in cryogenic environments less than 150K. This study evaluates the characteristics of composite materials such as tensile strength and strain using a UTM (Universal Testing Machine). The specimen is immersed in liquid nitrogen (77 K) to cool down during the test. The specimen is fabricated using 3D print, and can be manufactured by stacking reinforced fibers such as carbon fiber, fiber glass, and aramid fiber (Kevlar) with base material (Onyx). For the experimental method and specimen shape, international standards ASTM D638 and ASTM D3039 for tensile testing of composite materials were referenced.
Nonstop Monomer-to-Aramid Nanofiber Synthesis with Remarkable Reinforcement Ability
Koo, Jun Mo,Kim, Hojun,Lee, Minkyung,Park, Seul-A,Jeon, Hyeonyeol,Shin, Sung-Ho,Kim, Seon-Mi,Cha, Hyun Gil,Jegal, Jonggeon,Kim, Byeong-Su,Choi, Bong Gill,Hwang, Sung Yeon,Oh, Dongyeop X.,Park, Jeyoung American Chemical Society 2019 Macromolecules Vol.52 No.3
<P>Aramid nanofibers (ANFs), typically produced by exfoliating aramid microfibers (Kevlar) in alkaline media, exhibit excellent mechanical properties and have therefore attracted increased attention as nanoscale building blocks. However, the preparation of aramid microfibers involves laborious and hazardous processes, which limits the industrial-scale use of ANFs. This work describes a facile and direct monomer-to-ANF synthesis via an as-synthesized intermediate low-molecular-weight poly(<I>p</I>-phenylene terephthalamide) (PPTA) without requiring the environmentally destructive acids and high-order shearing processes. Under the employed conditions, PPTA immediately dissociates and self-assembles into ANFs within a time period of 15 h, which is much shorter than the time of 180 h (not including the Kevlar preparation time) required for the Kevlar-to-ANF conversion. Interestingly, the fabricated ANFs exhibit nanoscale dimensions and thermoplastic polyurethane (TPU) reinforcing effects similar to those of Kevlar-derived ANFs; i.e., a 1.5-fold TPU toughness improvement and a maximum ultimate tensile strength of 84 MPa are achieved at an ANF content of only 0.04 wt %. Remarkable reinforcement ability investigated by comprehensive analytical data comes from ANFs, which disturb ordered hydrogen bonding in hard segments and induce strain hardening along the elongation pathway. Thus, the developed approach paves the way to industrial-scale production of ANFs and related nanocomposites.</P> [FIG OMISSION]</BR>
VR Skywalking: 스카이워커 기구를 활용한 가상현실 운동 시스템
최혜진(Hyejin Choi),차호준(Hojun Cha),최승문(Seungmoon Choi) 한국HCI학회 2018 한국HCI학회 학술대회 Vol.2018 No.1
본 연구에서는 스카이워커(Skywalker, ROVERA) 운동기구와 HMD 를 결합하여 사용자가 게임을 하는 것처럼 즐기면서 운동 할 수 있는 VR 운동 시스템을 구현하였다. VR Skywalking 은 스카이워커 운동기구 위에서의 사용자 움직임을 감지하여 해당 정보를 VR 환경에 반영한다. VR 환경에서 사용자는 본 연구의 VR 운동 시스템 속 스토리 진행을 통해 다양한 게임요소를 경험하게 된다. 이를 통해 사용자는 실재감 넘치고 몰입도가 강한 가상현실을 경험할 수 있다.